Conventionally, as a technique for conducting analysis on substances, for example, there is a dual energy CT that applies two different types of tube voltages to an X-ray tube and conducts imaging with X-rays irradiated from the X-ray tube. When a substance is imaged by X-rays at an altered tube voltage, the substance exhibits a peculiar change in CT value. Based on this change in CT value, for example, contrast agents for blood vessels and calcified tissues may be analyzed.
An example of a dual energy CT is conducting imaging by means of two X-ray tubes by simultaneously irradiating one subject with X-ray from one X-ray tube with a high voltage applied and another X-ray tube with a low voltage applied.
Another example of a dual energy CT is imaging by using one X-ray tube to change the tube voltage while revolving X-ray tube, irradiating the subject alternately with X-rays of two types of tube voltages, high voltage and low voltage. Furthermore, the X-ray tube and an X-ray detector are arranged opposing one other and fixed to a revolving ring, with the revolving ring supported by a frame. In the following description, the revolution of the X-ray tube refers to an overall revolution of the revolving ring and the X-ray detector provided to the revolving ring, etc.
The X-ray generator, which generates X-rays by applying a tube voltage to the X-ray tube, is an inverter type to be smaller and lighter in weight. Regarding the X-ray generator with a tube voltage generator of an inverter type, the operating frequency component of the inverter inevitably tends to remain at the tube voltage. In such cases, the X-ray dosage emitted by the X-ray tube is also affected by the operating frequency component. Therefore, when altering the tube voltage during revolution of the X-ray tube, the above analysis is restricted by the timing of switching the tube voltage between high voltage and low voltage (tube voltage switching frequency) and the operating frequency of the inverter.
When the inverter operating frequency is sufficiently higher than the tube voltage switching frequency (for instance, 100 times or more), the inverter may be operated independently from the timing for switching the tube voltage. However, for example, if the tube voltage switching frequency is increased for cardiac diagnosis and the tube voltage switching frequency approaches the inverter operating frequency, there is a problem of declined reproducibility of the tube voltage waveform repeating the waveform at the same amplitude in any cycle. That is to say, there is a problem of the variation width of the tube voltage being different at every tube voltage switching cycle.
Next, this problem is described in detail.
The tube voltage switching frequency fs [Hz] may be expressed by the following formula.fs=R/(T*2)  (1)
Here, R is the collection rate, which is the number of X-ray images obtained per one revolution of the X-ray tube [view/rot], and T is the revolution time per one revolution of the X-ray tube [sec/rot].
For example in cardiac diagnosis, when R=900, T=0.35, the tube voltage switching frequency fs is 1285.71 [Hz] from the formula (1). Further, the tube voltage switching cycle Ts is 777.78 [μS].
Moreover, when the inverter repeats increase and decrease of the tube voltage, the clock number Nc required for a cycle period may be expressed by the following formula.Nc=Ts/Ti  (2)
Here, Ti is the operating cycle of the inverter [sec].
For example, in cardiac diagnosis, when the fundamental frequency fr of the inverter is 25 [kHz], the operating cycle of the inverter Ti is 40 [μs]. The clock number Nc calculated from the formula above (2) becomes 19.44.
However, since the inverter only operates in clock units in fact, the clock number Nc required for the cycle period may be 19 or 20. This dispersion of the clock number causes declined reproducibility of waveforms of the tube voltage.
FIG. 16 is a timing chart of an operating cycle (cycle of the clock) of the inverter and the timing for switching the tube voltage. As illustrated in FIG. 16, in the example mentioned above, when the tube voltage switching cycle Ts is 777.78 [μS] and the operating cycle Ti of the inverter is 40 [μS], the clock number Nc becomes 19.44.
FIG. 17 is a diagram illustrating the tube voltage waveforms. As illustrated in FIG. 17, the clock number Nc required for the cycle period may be 19 or 20 when the inverter repeats increase and decrease of the tube voltage. Therefore, the waveform of the tube voltage is dispersed.
As described above, when the tube voltage switching frequency is high upon cardiac diagnosis, etc., and is close to the inverter operating frequency, there is a problem of the reproducibility of the tube voltage waveform declining.
Moreover, a balancer is provided to the revolving ring provided with the X-ray tube, the X-ray detector and the X-ray generator, etc. Nevertheless, fluctuation appears while revolving. This fluctuation disadvantageously causes loss of X-ray images of high quality.
This embodiment is intended to provide an X-ray CT system that generates a tube voltage waveform with good reproducibility upon dual energy CT, so as to obtain high-quality X-images.